Bronx has an express train (the 6 i believe) that goes from Bx to Manhattan. I haven't been to NY in 3 years, but when i lived in the Bx, my commute to manhattan was much shorter than when i lived in BK, because of the 6 train. There are express trains in Brooklyn (Q?) but from where i traveled from it was still a long ride. Certain stations are just too major to skip over for an express train, so taking it was probably only a 5 to 10 minute difference from the regular train line (D or B). There'd be a bottleneck going over the bridge which would eventually cause all trains traveling that line to catch up to each other.